Electronic circuits are often encapsulated in a polymer material to protect the circuit from damage. A typical example of an electronic circuit is a common computer chip encapsulated before being mounted to a circuit board. Among other things, the encapsulation protects the chip from weather damage, electric charge, and dirt. Typical electronic circuits that are encapsulated are relatively small, having dimensions, for example, of about 1″×½″×⅛″. Further, typical encapsulated electronic circuits operate at most at a few hundred volts.
Encapsulating the electronic circuit is often accomplished by various known techniques, including flowing a mixture of an encapsulation material combined with a curing agent over the electronic circuit and allowing the mixture to solidify. By itself, the encapsulation material remains liquid. The curing agent is added to foster a chemical reaction that makes the mixture harden. The hardening process happens in a relatively short period of time. Because the electronic circuit is small, the mixture does not harden in the time it takes to cover all areas of the circuit. Because the electronic circuit operates at relatively low voltages, variations in the electrical material properties of the encapsulation material are acceptable.
Problems arise in the encapsulation process, however, when the electronic circuit is large and/or when the electronic circuit operates at very high electric fields. Some examples where problems arise are high voltage devices, such as power supplies, that operate at many tens of thousands of volts and comprise multiple electronic circuits and devices all packaged together to form an assembly. In these cases, encapsulation becomes critical. In particular, the encapsulation material must be able to withstand the high voltages to which the assembly is exposed. Also, the solidified encapsulating material should be free of voids and air bubbles in order to sustain such high voltages. Variations in the material properties become unacceptable as this can cause catastrophic failure to the assembly and the surrounding components.
Moreover, manufacturers do not make silicone encapsulation materials capable of flowing long enough to accurately cover large structures such as a 6″ by 4″ by 2.5″ assembly mounted in a potting enclosure. Other encapsulation materials, such as epoxies and urethanes, etc. may provide ample flow time. However, each material has its own limitations, such as hardness and/or dielectric strength that can make them unacceptable for encapsulating large structures, such as electronic circuits, that operate at very high electric fields. As such, design engineers are forced to choose which limitations to accept. In the case of conventional silicone encapsulation materials, one problem is that the curing agent causes the mixture to have improper flow characteristics or harden long before large assemblies can be accurately and completely encapsulated. Typically what happens when attempting to encapsulate a large assembly is that the mixture covers only a part of the assembly before loosing its flowability, which can create voids and trap air bubbles. The remaining portion of the assembly is either not encapsulated or the material properties of the encapuslant have large variations throughout.
In the past, the amount of curing agent has been adjusted in an attempt to extend the flow time of the mixture. However, this has not solved the problems associated with variations in the material properties of the encapsulation material.
Problems with variations are exacerbated when the assembly is used in extreme conditions, such as onboard aircraft and marine craft. In the case of aircraft, the assembly can be exposed to extreme temperature, pressure, and moisture conditions. Combining these conditions with a high voltage device leads to unacceptable encapsulations schemes.
Thus, there is a need for a method and a material to encapsulate an assembly and, during the curing process, determine the dielectric strength of the silicone so as to adjust the curing process accordingly and provide for the required dielectric strength.